What Age Should Kids Stop Trick Or Treating?

With Halloween around the corner, many people are busy getting decorations out and preparing for the onslaught of trick-or-treaters.  And no matter which end of the candy you're on - I think everyone has posed this question before:  What age should kids stop trick-or-treating?  It always seems like there is a grey-area that surrounds this perennial Halloween question.

A city in Canada has aimed to answer the question once and for all;  Bathurst, New Brunswick recently established 16 as the cut-off year and also arranged a curfew of 8:00 PM.  The penalty?  A fine of $200.

What are your thoughts?  What age did you stop trick-or-treating?  Or - if you stay home and hand out candy, have you ever had someone knock on your door that you've taken issue with?